If the units of dark matter and dark energy are different (which I guess they are) then how can the numerical quantities of each be compared with each other? I know that dark energy is sometimes referred to as a "force," but energy does not have the dimensions of a force or a mass. How is it possible to compare "things" percentage-wise if they don't have the same physical units?

There are a couple of ways that these can be compared. Einstein showed that matter can be converted to energy, the conversion factor being the speed of light squared. If you do this conversion for dark energy and dark matter then you find that approximately 70% of all the matter/energy in the Universe is in dark energy, about 25% is in dark energy, and the rest is in normal matter and radiation.

Another way to perform the comparison is to look at how they impact the acceleration of the expansion of the Universe. Energy is equivalent to matter in the sense that it gravitates, and acts to slow the expansion. Dark matter therefore acts purely to decelerate the expansion of the Universe. The energy density associated with dark energy also acts to decelerate the expansion, but the main effect of dark energy is that it has a negative pressure that accelerates the expansion. This means that for a given amount of either dark energy or dark matter the impact of the dark energy is a factor of 2 greater on the acceleration of the expansion of the Universe. So given their relative abundances at the present day, the impact of dark energy is about 5.5 times stronger on the acceleration of the expansion.

About the Author

Mike is a fourth year astronomy graduate student at Cornell, where he works with Professors Martha Haynes and Riccardo Giovanelli on the ALFALFA survey, a blind survey of gas-rich galaxies in the local Universe carried out with the 305m Arecibo telescope in Puerto Rico.

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